The fuel cell is an apparatus that converts chemical energy of a fuel directly into electrical energy by no way through mechanical energy nor thermal energy, and is high in efficiency of power generation, and has, as the next-generation of power generating apparatus, great hopes put thereon.
As a fuel cell to be mounted on an automobile, a polymer electrolyte fuel cell using an ion exchange membrane is watched. For the polymer electrolyte fuel cell, the basic configuration and actions will be described.
The polymer electrolyte fuel cell is configured as a complex cell that has a plurality of laminated simplex cells (herein sometimes referred to as “single cells”) to be fundamental units for power generation.
Each of the single cells has an MEA (membrane electrode assembly) that has a fuel electrode or positive electrode (hereinafter referred to as “anode”) and an oxidant electrode or negative electrode (hereinafter referred to as “cathode”) interposed on both sides of a solid polymer electrolyte membrane, respectively. Further, the single cell has an anode side separator and cathode side separator provided with gas channels and cooling water channels, outside the anode and the cathode, respectively.
The anode has a catalytic layer outside the solid polymer electrolyte membrane, and has a fuel gas diffusion layer outside it. The cathode also has a catalytic layer outside the solid polymer electrolyte membrane, and has an oxidant gas diffusion layer outside it.
In the polymer electrolyte fuel cell, a gaseous fuel (herein sometimes referred to as “fuel gas”) containing hydrogen is supplied to the anode, where reactions of the following expression (1) occur in the catalytic layer, and a gaseous oxidant (herein sometimes referred to as “oxidant gas”) containing oxygen is supplied to the cathode, where reactions of the following expression (2) occur in the catalytic layer.H2→2H++2e−  (1)½O2+2H++2e−→H2O+Q (reaction heat)  (2)
Therefore, every single cell of the fuel cell apparently has a reaction of the following expression (3) progressing therein.H2+½O2→H2O+Q  (3)
This reaction accompanies a necessary electromotive force for movement of electron (e−), which can be taken outside in the form of electrical energy.
As will be seen from the expression (1), the anode has hydrogen ions (protons) generated in the catalytic layer, which hydrogen ions move to the gas diffusion layer in the cathode via proton exchange groups in the solid polymer electrolyte membrane as a transmission medium. Proton exchange groups in the solid polymer electrolyte membrane have a decreased specific resistance as the electrolyte membrane has a saturating moisture content, acting as a proton-conductive electrolyte. Therefore, in order to keep the solid polymer electrolyte membrane in a water containing state, the reaction gas to be supplied to each single cell is humidified in advance. In each single cell, the solid polymer electrolyte membrane is thereby allowed for a suppressed evaporation of the moisture, with a resultant protection of the drying.
Further, as will be seen from the expression (3), the cathode has water produced in the catalytic layer as a power generating reaction is advanced in the fuel cell, and the produced water flows downstream in each single cell, together with oxidant gas. Therefore, by concurrent presence of such water that has been contained in oxidant gas for humidification of the solid polymer electrolyte membrane and such water that has been produced along with the power generating reaction, each single cell may tend to have an increased content of moisture residing in the downstream region. Thus, there is a possibility that this region may be over-saturated, generating droplets, and impeding a favorable diffusion of oxidant gas.
To this point, the oxidant gas to be supplied may have a reduced content of moisture for humidification to effect a decrease in total amount of residual moisture in the downstream region of each single cell, which may however be accompanied by a raised utilization of oxidant gas to increase the efficiency of power generation, yet with the possibility of producing much water in the catalytic layer, causing an over-saturation, generating droplets.
Accordingly, in each single cell, the catalytic layer may have a carbon carrier of a porous planer or particle shape carrying a platinum catalyst, and an intervenient electrolyte (e.g. PTTF, etc.) for provision of a water repellency thereto, to thereby prompt draining produced water or condensed water.
In addition, as will be seen from the expression (1), the fuel cell has in the startup a process of supplying a hydrogen gas as the fuel gas to the anode, where the anode may have H2 and residual air mixed in the upstream and the downstream, forming to the anode a local cell (with an upstream anode and a downstream cathode). Then, the solid polymer electrolyte membrane neighboring the anode may have a deficient state of hydrogen ion at the downstream, with a resultant gradient of hydrogen ion concentration causing the solid polymer electrolyte membrane to have a lowered potential in the downstream. As a result, the solid polymer electrolyte membrane may have an increased potential difference to the catalytic layer at the cathode side, which may be accompanied by occurrences of such a corrosion of carbon carriers as shown by expressions (4) and (5) and such a melting of Pt as shown by an expression (6), in the catalytic layer at the cathode side.C+2H2O→CO2+4H++4e−  (4)C+H2O→CO2+2H++2e−  (5)Pt→Pt2++2e−  (6)
Such phenomena may occur in a start of the fuel cell, as well as in a stop, with a yet accelerated tendency along repetition of start and stop operations of the fuel cell. Thus, there is a possibility that the performance of power generation may be reduced as the cell voltage decreases.
With such points in view, for the cathode's catalytic layer as a factor to determine the performance of power generation, besides the drainage, it has been desired to suppress the catalyst's activity reduction due to (platinum) catalyst elution and carbon carrier corrosion.
For an enhanced anti-corrosiveness of a carbon carrier, Japanese Patent Unexamined Publication No. 2005-26174 has disclosed a cathode catalytic layer, in which the carbon carrier has an increased degree of graphitization, and the specific surface area as well as the bulk density is set within a specified range.
On the other hand, for an enhanced activity of a platinum catalyst, Japanese Patent Unexamined Publication No. H6-150944 has disclosed an electrode, in which the catalytic layer is double-layered, and a catalytic layer at the solid polymer electrolyte membrane side has a more increased amount of platinum catalyst than that at the gas diffusion layer side. Further, Japanese Patent Unexamined Publication No. H6-103982 has disclosed a fuel cell in which for a double-layered catalytic layer, in a catalytic layer at the solid polymer electrolyte membrane side, the amount of electrolyte is increased, or the amount of platinum catalyst is increased more than that in a catalytic layer at the gas diffusion side of electrode. In addition, in Japanese Patent Unexamined Publication No. H11-312526, there has been disclosed even an electrode in which, for a double-layered catalytic layer, the particle size of metal catalyst in a catalytic layer at the gas diffusion side of electrode is set as greater as 1.5 times or more than the particle size of metal catalyst in a catalytic layer at the solid polymer electrolyte side.